Class 9 Worksheet- 3. Atoms and Molecules

3. Atoms and Molecules

1. For question numbers 1 to 7, two statements are given-one labelled Assertion (A) and the other labelled Reason (R).

Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below :

(a) Both A and R are true and R is the correct explanation of the assertion.

(b) Both A and R are true but R is not the correct explanation of the assertion.

 (c) A is true but R is false.

(d) A is false but R is true.

1. Assertion (A) : During a chemical reaction only few of the reactants react to get converted Into products.

Reason (R) : During a chemical reaction total mass of the reactants is equal to the total mass of the products.

2. Assertion (A) : Hydrogen and oxygen combine in the ratio 1:8 by mass to form water.

 Reason (R) : Water taken from different sources has the same ratio of H' and O, i.e., it is always '1:8’.

 3. Assertion (A) : All matter is made up of very tiny particles called atoms.

 Reason (R) : Physical and chemical methods are available to separate matter consisting of atoms.

 4. Assertion (A) : A molecule is the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.

 Reason (R) : A molecule is a group of two or more atoms that are chemically bonded together, Le., the atoms are tightly held together by attractive forces.

5. Assertion (A) : The chemical formula of a compound is a symbolic representation of its composition.

Reason (R) : The simplest compounds, which are made up of two different elements are called binary compounds.

6. Assertion (A) :1 mole of atoms of an element has a mass equal to gram atomic mass of the element.

Reason (R) : A group of 6.022 x 10²³ particles (atoms, molecules or ions) of a substance is called a mole of that substance.

7. Assertion (A) : An ion may be defined as an atom or group of atoms having positive or negative charge.

Reason (R) : An ion is formed by either loss or gain of electrons.

II. Glve one word for the following : (1 Mark]

1. The law that states, "Mass can neither be created nor destroyed."

2. The law that states, "A pure chemical compound always contains the same elements combined together in the same proportion by mass irrespective of place from where the sample has been taken".

3. The theory, that states that atoms are indivisible particles, which can't be created or destroyed in a chemical reaction.

 4. The symbolic representation of the composition of a compound obtained by cross-multiplying their valencies.

5. Positively charged ions

6. Negatively charged ions.

7. The mass of 1 mole of a substance and is equal to Avogadro's number.

8. Compound formed by the combination of a metal with a non-metal.

9. Latin name and symbol of iron. 10. An ion that contains group of atoms.

III. Fill in the blanks : (1 Mark]

 1. Law of chemical combination was given by ____________ pure ____________.

2. The law of ____________states that mass can neither be created nor destroyed in a chemical reaction.

3. STM stands for ____________ .

 4. IUPAC that stands ____________  for approves the names of elements.

 

 5. A/ an _____________is the smallest particle of an element which takes part in chemical

reaction.

 6. _____________ is the quantity of mass equal to 1/12 of mass of an atom of carbon-12.

 7. A _____________ is the symbolic representation of the composition of a compound.

8. One atomic mass unit is equal to_____________ gram.

. 9. Atomicity of solid sulphur, solid phosphorus and sodium are_____________ , _____________ , _____________respectively

10. The molar mass of a substance is the mass of 1 mole of that substance that is equal to the _____________atoms of that element substance.

11.In water, the ratio of the mass of hydrogen to the mass of oxygen is always_____________ respectively, whatever be the source of water.

12. Formula of aluminium sulphate is _____________.

13. The Avogadro constant 6.022 x 10²³ is defined as the number of atoms in exactly of carbon-12.

 14. Mass of 1 mole of a substance is called its _____________.

15. A _____________ is the smallest particle of an element or a compound capable of independent  existence under ordinary conditions.

IV. State whether the following statements are True or False :

1. Law of conservation of mass states that during a chemical reaction total mass of reactants will be equal to total mass of products.

2. One mole ofH₂CO₃, contains 6.023 x 10²³ atoms of hydrogen.

3. Atoms of most elements are very reactive and do not exist in free state.

 4. Mass of one molecule of water is 18 g.

5. Molecular mass is the sum of atomic masses of all the atoms in a molecule of that substance. 6. SO₄^2- is called sulphide ion.

7. The formula unit mass of CaCl₂, is 100 g (When Ca = 40 g. C- 35.5 g).

 8. In 1961, IUPAC has accepted 'atomic mass unit' (u) to express atomic and molecular mass of elements and compounds.

 9. According to Dalton's atomic theory, atoms of an element have identical mass and chemical properties.

10. Law of constant proportion states that a pure chemical compound always contains the different elements combined together in the same proportion by any mass.

11. Chemical formula of calcium sulphate is CaSO₄.

12. H₂O, CO₂, NaCl, CaCO₃, etc. are the best examples of heteroatomic molecules.

13. As per the scale 1 nm = 10 ^-12m.

14. While writing the chemical formula, if a compound is formed of a metal and a non-metal, symbol of the metal is written first.

15. Collection of 6.022 x 10²⁵ particles (atoms, molecules or ions) of a substance is called a mole of that substance.

 V. Match the terms given in column I with those given in column II. [I Mark)

 

 

Column I	Column II
1. Latin name of sodium	(a) 'atoms' constitute matter
2. Latin name of potassium	(b) ½ x mass of an atom of C12
3. Law of constant proportion	(c) S8
4. Dalton's atomic theory	(d) natrium
5. Formula of sulphur	(e) collection of 6.022 x 1023 entities
6.1u	(f)  elements are always present in definite proportions by mass
7. NaCl	(g) kalium
8. 1 mole	(h) ionic compound
9. H2O	(i)(NH4)2 SO4
10. Chemical formula of ammonium sulphate	(j) 2 x 6.022 x 1023 atoms of H and             6.022 x 1023 atoms of oxygen

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 Answer